In the UTRAN architecture of MBMS in 3GPP Release 6, as combining methods of MBMS transport channels (MTCH), two main types are defined as selection combining and in-phase combining (soft combining) (refer to 3GPP TS25.346, for example).
These combining methods are used according to environment, in which the selection combining is normally performed in an environment of Inter-Node B, that is, among base stations, and the soft combining is normally performed in an environment of Intra-Node B, that is, among sectors. The reason is that the system architecture of Release 6 is an asynchronous system.
However, in a future mobile communication system Evolved-UTRAN (E-UTRAN) for which discussions are started in 3GPP(3rd generation partnership project), synchronization among base stations are assumed as the system architecture. For example, based on synchronization by GPS (Global Positioning System), it is assumed to aim high performance by performing soft combining on MBMS data from adjacent cells among base stations.
In this case, since it is necessary to transmit a same signal among adjacent base stations, that is, in Inter-Node B, data loss on a transmission route to the base stations becomes a large problem. To solve the problem, it is necessary to detect frame loss in an interface between a base station (Node B) and a radio network control apparatus (RNC: Radio Network Controller) or between a base station and a MBMS server.
In MBMS transmission of Release 6, since soft combining is defined only in Intra-Node B, that is, only among cells (sectors), such problem does not occur. Under a same base station (Node B), since MTCH that is transmitted in the cells (sectors) is the same, frame loss becomes the same among all cells (sectors). That is, the frame loss is closed in Node B.
In addition, since selection combining is performed in Inter-Node B, that is, among base stations, frame loss between the base station and the radio network control apparatus is not a problem. This is because, even though there is a frame loss in one Node B, there is no problem if another one can receive the frame.